You don't let terrorist leaders just flee. These people need to be tracked down and brought to justice. Otherwise, they will just metastasise wherever they end up and you'll have the same problem in a couple of decades again. Going after the financial networks and the support structures in universities will be more important than the low level "organisers" though. But, those low level organisers have information and can be made to testify (unless they like the prospect of spending the rest of their lives in federal prison) against their bosses.